Piezoelectric devices generally include at least quartz resonators and quartz filters. Both of these types of devices are typically constructed using a thin plate of piezoelectric quartz material. This thin plate is usually cut from a larger piece of quartz with a certain angular orientation of the cut with respect to the crystalline lattice structure of the quartz to produce a plate that will vibrate in some predetermined desired mode. (So-called GT-cut crystals for instance vibrate in a so-called face-shear mode, which is a vibration mode that has a non-vibrating nodal area at the center of the plate.)
The thin plate of piezoelectric quartz has opposing surfaces to which are bonded electrically conductive materials and upon which an electric signal is impressed that causes the quartz plate to vibrate. At one or more particular frequencies, the input impedance of a quartz resonator, or filter, (as seen looking into the electrodes) is minimized and this frequency (or frequencies) is the resonant frequency of the resonator or filter.
In many respects, a quartz filter or resonator are mechanical devices in that electrical signals coupled to a pair of input electrodes coupled to the quartz plate induce mechanical vibrations in the plate. In a quartz filter, the vibrations induced in the crystal from an input signal are transmitted through the crystalline structure of the quartz to an output resonator stage, which is also comprised of output electrodes bonded to the surface of the piezoelectric plate, whereat the mechanical vibrations induce electrical signals on the output electrodes. Most crystal filters permit only a relatively narrow band of frequency signals on the input electrodes to pass through the crystal to the output electrodes and as such, a quartz filter acts like virtually any other electrical filter constructed from passive circuit elements such as resistors, inductors, and capacitors except that a quartz filter is constructed from a thin plate of dielectric material.
A quartz resonator, like a quartz filter, is also a mechanical device in that electrical signals coupled to a pair of input electrodes coupled to the quartz plate induce mechanical vibrations in the plate. As in a quartz filter, physical characteristics of the quartz plate, such as its thickness, length, width, temperature, cut orientation, etc., will effect a relatively low input impedance to a driving signal source at the resonant frequency of the plate. (The resonant frequency of the plate is a frequency at which the plate most naturally vibrates. A quartz plate might have several different resonant frequencies including overtones, harmonics.) Because the crystal presents a high impedance away from a resonant frequency, a quartz resonator can be used to control an oscillator circuit.
In fabricating a piezoelectric resonator or a piezoelectric filter, the frequencies of its operation can be affected by many factors including the thickness of the quartz, the orientation of the crystal lattice structure of the quartz with respect to the directions in which it is cut and the dimensions of the electrodes bonded to the surfaces of the quartz material. The resonant frequency can be changed by changing the mass loading on the piezoelectric quartz, which can be accomplished by either decreasing or increasing the thickness of the electrodes. In a quartz filter, characteristics such as the bandwidth, Q, and so forth, can also be adjusted by changing the mass loading of the electrodes on the surfaces of the piezoelectric quartz.
Prior art methods of tuning a piezoelectric device, such as a resonator or filter, generally use a vacuum deposition process in which atoms of electrode material are added to the electrodes while the electrical characteristics of the device are monitored. Material is typically added to the electrodes using a vacuum deposition chamber and a so called finish plate mask, which is simply a template or mask having openings or windows aligned with and placed over the electroded areas of the quartz. The mask is usually physically separated from the quartz by some distance away from the surface of the quartz. Atoms that pass through the windows in the mask during the tuning process attach themselves to the surfaces of the electrodes on the quartz to increase the thickness of the electrodes and increase the mass loading of the electrodes on the quartz, decreasing the resonant frequency of the plate/electrode combination.
Alternate methods of tuning a piezoelectric device might remove material from electroded areas, reducing mass loading and increasing resonant frequencies. For a variety of reasons, adding material to electrodes is preferred over removing material, one reason being the difficulty in uniformly removing material from an existing, planar electrode compared with the relative ease of uniformly adding material using vapor deposition techniques.
A problem with prior art tuning methods that add material using a finish plate mask is that the physical registration, or alignment, between the finish plate mask and the electrodes on the surfaces of the piezoelectric plate must be carefully controlled. Any misalignment between the windows or openings in the finish plate mask with respect to the electrodes, can cause a widening or a shifting of the area of the electrode effecting other electrical characteristics of the device.
Even after a piezoelectric device is tuned using the prior art method of using a finish plate mask, a second problem with the prior art piezoelectric devices is the subsequent fixturing or attachment of the tuned piezoelectric device to a substrate by which it might be mounted to a circuit board. Much effort has been spent attempting to miniaturize electrical components using so called surface mount techniques. Mounting a tuned piezoelectric device to a substrate or carrier that is suitable for surface mount manufacturing presents a significant problem because of the physical dimensions of quartz devices, particularly those with resonant frequencies in the megahertz frequencies.
A third problem with prior art piezoelectric devices is that the quartz material comprising a piezoelectric device must be compliantly mounted to its substrate so that mechanical stresses on the quartz, from either thermal effects or mechanical shock, are minimized. Rigidly attaching a piezoelectric device to any type of substrate invites mechanical stresses in the piezoelectric as its temperature changes. Mechanical vibrations, including mechanical shock can also be transmitted into the quartz plate if it is not properly mounted. Mechanical stresses can affect the resonant frequency or the frequency of operation significantly and many techniques for compliantly mounting a piezoelectric have been suggested.
Yet another problem with prior art piezoelectric devices is electromagnetic shielding some of these devices might require. Strong electromagnetic fields can also adversely affect the operation of a piezoelectric device. Since these devices may require shielding from electromagnetic fields, providing such shielding frequently requires their enclosure into a volume enclosed by metallic or conductive material.
A device that accommodates all of these prior art difficulties, including the compliant mounting of the quartz, precise alignment of a finish plate mask during manufacturing but yet one that preferably does not have to be removed after adjustment, surface mounting of the finished device on to a substrate, and the RF shielding, while providing a signal path to the device, would be an improvement over the prior art.